Update return values of vector client plugins to return consistent results

Problem/Motivation

We currently pass back results as is from the API response. However, this then means that for example:

1. Pinecone returns and array of 'matches'
2. Milvus returns an array of 'data'

Steps to reproduce

Proposed resolution

Normalise the results to have a consistent format across these and future plugins.

Remaining tasks

Update interface and plugins

User interface changes

None

API changes

Updated plugins & interface for vector database clients.

Data model changes

Returning a normalised response from the API.